The present invention relates to control systems for nuclear power plants, and in particular, to control systems which respond to signals indicative of the position of control rods in the reactor core.
In a typical pressurized water nuclear power reactor, a plurality of control element assemblies are supported on the nuclear reactor vessel with associated drive mechanisms for moving the control element assemblies into and out of the reactor core, for the purpose of controlling the gross power level, or the power distribution in the core. Typically, each control element assembly (CEA) has a shaft which is driven by a plurality of electromagnetic coils having latches which engage grooves on the shaft, whereby the shaft may be moved in stepwise fashion. The shaft terminates in a web or similar structure, which supports a plurality of individual control elements or rods sized to pass through openings within or between fuel assemblies which define the reactor core.
A particular nuclear reactor may have dozens of CEA's, which are typically grouped or ganged in sets of four or eight, so as to move in unison when actuated by respective drive mechanisms. Moreover, these groups may be programmed to move in an overlapping sequence, with some of the groups not actuable for normal operational power control, but rather held in reserve only in the event of a reactor trip, whereby all CEA's are disengaged from the drive mechanisms, thereby dropping under the force of gravity into the reactor core to shut down the reactor power as quickly as possible.
The generation of a reactor trip signal typically occurs in a safety control system having an algorithm which takes into account the position of the CEA's in the reactor core, in combination with many other operational variables, to assure that if the operation of the installation is expected to pass through the boundary of the permissible operating envelope, the reactor will trip.
The insertion position and movement of each CEA is typically sensed by a reed switch configuration, each having a position output signal that is sent to the safety control system. The signal is used within the safety control system as part of a calculation to determine if the reactor should be tripped. Spurious changes in a CEA position signal, not caused by actual CEA movement, can cause unnecessary trips of the reactor. Inasmuch as the trip shuts down all power generating capacity from the affected reactor, the disruption to plant operation is self-evident. At a more subtle level, however, spurious CEA position signals, can result in the safety control system determining that the reactor is operating near a boundary of its operating envelope (i.e., with low safety margin), thereby unnecessarily restricting the operator from implementing the most cost-effective system configuration or power maneuvering.